Tris(2,3-dibromopropyl)isocyanurate (“TDPI”) is a very effective flame retardant with high thermal stability. It is used mainly to provide flame retardancy to polypropylene and also to polyethylene and polystyrene. Advantages of TDPI are its low viscosity, low tendency towards discoloration and low corrosivity in relation to process tooling.
The first process for the preparation of TDPI was described in 1972 in DE 2244543. This application describes the synthesis of brominated triallyl isocyanurate [CAS No. 1025-15-6], using dichloromethane as solvent. After bromine has been added, the reaction product is precipitated via addition of petroleum ether, and filtered. The filter cake is triturated with MeOH and washed with water. This procedure—reaction in a, preferably, chlorinated solvent and precipitation of TDPI via addition of a non-solvent—is also described in many more recent patent applications.
JP 2000053658 decribes a proceedure in which triallyl isocyanurate is brominated in dichloromethane and methanol is used as a precipitatant. JP 1999228549 discloses a reaction medium composed of a mixture of heptane and dichloromethane and in which heptane is used for precipitation.
JP 2000053658 mentions not only dichloromethane but also reaction solvents such as 1,1,1-trichloroethane, chloroform, carbon tetrachloride, ethylene dichloride, dibromomethane and others. Among the precipitants listed are: methanol, ethanol, isopropanol, pentane, hexane, cyclohexane. In order to remove excess bromine (colour reduction) the reaction solution is also treated with an aqueous hydrazine solution.
In all of these processes, fundamental problems arise from the physical properties of TDPI and from the chemical/physical properties of the precipitants. Although TDPI has a melting point of greater than 100° C., it tends to precipitate in liquid form when precipitated from solutions via addition of a non-solvent. This liquid phase solidifies after some time and becomes a compact solid product. The only process technology which can change this behaviour is the use of very heavy-duty stirrer systems, e.g. screw kneaders. Normal stirrers such as those used in stirred tanks are insufficient and cease to operate. Precipitation of TDPI in a stirrable, crystalline and filterable form, without passing through the liquid phase, requires a very sophisticated process of precipitant addition.
A second difficulty is that there are very few remaining chlorinated hydrocarbons that can be used industrially. One of these is dichloromethane. If the system used comprises dichloromethane as reaction solvent and methanol as precipitant, distillative separation becomes impossible because an azeotropic system forms; its constitution being 93% of CH2Cl2; 7% of CH3OH, with a boiling point of 38° C. This mixture cannot, moreover, be used as solvent for subsequent batches because the methanol reacts with bromine and liberates highly corrosive hydrogen bromide.
Petroleum spirit or heptane also reacts with bromine, and an essential requirement of the process therefore has to be clean distillative isolation of the precipitant from dichloromethane. Experience has shown that the content of precipitant in the dichloromethane should not exceed 0.1%.
It has also been found that repeated use leads to an increase in the concentration in the precipitant of by-products which, on standing, give tacky deposits and require repeated cleaning of containers, and also distillation of the actual precipitant.
A process which takes into account the problems described above should therefore have the following features:                reaction of triallyl isocyanurate with bromine in dichloromethane        addition of a precipitant which does not form an azeotrope with dichloromethane        crystallization of TDPI        filtration and washing (with precipitant)        drying        separation of dichloromethane from precipitant by means of multistage distillation        distillation of precipitant as required by degree of contamination        discarding of by-products.Finally, the large-scale preparation of TDPI is relatively expensive and a more cost effective procedure would be highly desireable.        